Dynamic illumination control for laser projection display

ABSTRACT

A method for improving image contrast for a display apparatus obtains a frame of image data having one or more code values for each pixel in the frame of image data, analyzes the frame of image data to identify a distribution of dark regions therein. The method further adjusts at least one of the one or more code values for the frame of image data according to the distribution of dark regions in the frame of image data and attenuates a brightness level available for the image frame according to the distribution of dark regions in the frame of image data.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned U.S. patent application Ser. No.12/484,374 (now U.S. Publication No. 2010/0315595), filed Jun. 15, 2009,entitled DYNAMIC ILLUMINATION CONTROL FOR LASER PROJECTION DISPLAY, byMarcus et al., the disclosure of which is incorporated herein

FIELD OF THE INVENTION

The present invention relates to electronic display apparatus usingspatial light modulators and more particularly relates to apparatus andmethods for improved contrast and dynamic range in an electronicprojection system.

BACKGROUND OF THE INVENTION

A critical performance measure in image projection systems is thecontrast ratio (C/R) which represents the light intensity differencebetween the brightest white and the darkest black. C/R is thus definedby the relationship:C/R=white luminance/black luminance  (1).Improving the C/R helps to provide better on-screen image reproduction.

In general it is difficult to increase C/R by increasing the whiteluminance of the projection system since most projectors are lightsource limited. Thus approaches to improve the contrast ratio attempt todecrease the black luminance level. The black luminance level is aresult of light that passes through the active display devices in theoff state and stray light coming through the projection optics of thedisplay. Stray light can come from unwanted reflections from opticalcomponents of the projection system.

There have been a number of proposed approaches to improving thecontrast ratio of an electronic display. For example, in U.S. Pat. No.7,413,314 (Kim et al.) describes an optical system having an iriscontrolled in real time for reducing light from devices in the offstate. In the '314 optical system, an iris controller senses luminanceinformation in the light output and controls the projection irisaccording to the luminance information. With the opening range of theiris controlled in real time, the contrast ratio (C/R) is improved.

U.S. Pat. No. 7,204,594 (Akiyama) describes a projector including anillumination device, an electro-optic modulator, and a projectionoptical system that includes a light shielding member provided with astray light elimination member that reflects unwanted light away fromthe projection optics path.

Another approach has been to modify the display screen itself. Forexample, U.S. Pat. No. 7,403,332 (Whitehead et al.) describes a displayhaving a screen incorporating a light modulator which is illuminated bya light source composed of an array of controllable light emitters. Thecontrollable emitters and elements of the light modulator may becontrolled to adjust the intensity of light emanating from correspondingareas on the screen.

Each of these conventional approaches for contrast ratio improvement hasits shortcomings. The mechanical iris of the '314 disclosure must be ahigh-speed device and can be relatively costly. The light-shieldingmember of the '594 disclosure sends stray light out of the projectionpath and to other surfaces inside the projector, with the potential forsome portion of this light to be projected onto the screen. Thespecialized display screen taught in the '332 disclosure addssignificantly to projection system cost and may not be a suitablesolution where it is desirable to replace existing film projectionequipment.

Of particular interest are solutions that are appropriate for projectionsystems that use laser light sources. These can include, for example,systems that use spatial light imaging modulators such as liquid crystaldevices (LCDs) or digital micromirror devices, such as the DLP devicefrom Texas Instruments, Inc., Dallas, Tex.

Another type of imaging modulator device that is well-suited for usewith laser sources are linear light modulators. Linear light modulatorsform images by a rapid, repeated sequence in which each single line ofthe image is separately formed and is directed to a screen or otherdisplay surface by reflection, or other type of redirection, from ascanning element. Types of linear light modulators that operate in thismanner include devices such as grating light valves (GLV) from SiliconLight Machines and described in U.S. Pat. No. 6,215,579 (Bloom et al.),and elsewhere. Display systems based on GLV devices are disclosed, forexample, in U.S. Pat. No. 5,982,553 (Bloom et al.). Another type oflinear light modulator is the piezoelectric based spatial lightmodulator (SOM) developed by Samsung and disclosed, for example, in U.S.Pat. No. 7,133,184 (Shin et al.).

An improved type of linear imaging modulator is the gratingelectro-mechanical system (GEMS) device, as disclosed incommonly-assigned U.S. Pat. No. 6,307,663 (Kowarz), and elsewhere.Display systems based on a linear array of conformal GEMS devices aredescribed in commonly-assigned U.S. Pat. Nos. 6,411,425, 6,678,085, and6,476,848 (all to Kowarz et al.). Further detailed description of GEMSdevice architecture and operation is given in a number ofcommonly-assigned U.S. patents and published applications, includingU.S. Pat. Nos. 6,663,788 (Kowarz et al.), and 6,802,613 (Agostinelli etal.). In these devices, light is modulated by diffraction. On a GEMSchip, for example, a linear array of conformal electromechanical ribbonelements, formed on a single substrate, is actuable to provide one ormore diffracted orders of light to form each line of pixels forline-scanned projection display.

Color display system architectures using LCD, DLP, GLV, SOM, and GEMSdevices generally employ three separate color paths, red, green, andblue (RGB), each color path provided with a separate spatial lightmodulator and laser source. When actuated, the light imaging modulatormodulates its component red, green, or blue laser light to form theimage, a single frame of pixels or line of pixels at a time. Theresulting modulated frames of pixels or lines of pixels for each colorare then combined onto the same output axis to provide a full-colorimage that is then directed onto the display screen.

Linear light imaging modulator arrays have exhibited some advantagesover their area array spatial light modulator (SLM) counterparts byvirtue of higher resolution, reduced cost, and simplified illuminationoptics. GLV and GEMS devices are actuable to operate at fast switchingspeeds for modulating laser light. GLV and GEMS devices have advantagesfor high resolution, high native bit depth, variable aspect ratio, andrelative freedom from motion artifacts when compared against other typesof spatial light modulators.

However, there are a number of limitations inherent to linear spatiallight modulators that can tend to constrain projector performance. Anumber of limitations relate to the scanning sequence itself. Thegalvanometrically actuated scanning mirror that is conventionally usedto scan modulated light across the display surface rotates over a shortangular range to form each 2-D (two-dimensional) frame of the image.Following each scan, mirror position must then be reset into thestarting position for the next scan. During this reset interval, imagecontent is not projected, when using the standard scanning sequence.Thus, light output is not available during about 15-25% of the operatingcycle, since the mirror requires some amount of time to stop, reversedirection, and return back into position for the next scan. Thisinherent reduction of the available light output limits the lightefficiencies that can be obtained. Due to this scanning mirror resettime and to acceleration and deceleration times of the mirror, theeffective duty cycle for providing modulated light with such systems,the so-called “pixel on” time, is typically no more than about 72-85%.

Another problem related resulting from the scanning sequence relates tothe need to minimize the effects of stored charge as the ribbon elementsare repeatedly switched between positions. Electrostatic energy is usedto actuate the ribbons. Maintaining the same charge polarity for theintegrated circuit (chip) substrate from one scan to the next quicklybuilds up a residual charge in the device that must be compensated foror dissipated in some way. In response to the problems of chargebuild-up, commonly-assigned U.S. Pat. No. 6,144,481 (Kowarz et al.)discloses a method for correcting for charge accumulation in the spatiallight modulator device. This method applies, to the dielectric ribbonelements, a modulated bipolar voltage signal whose time average is equalto the time average of a bias voltage applied to the bottom conductivelayer of the modulator device. The resulting alternating waveformswitches the polarity of the substrate bias voltage effectivelycanceling the charge build-up during operation of the device.

Although the method described in the Kowarz et al. '481 disclosurecorrects for problems related to charge build-up, transient movement ofthe modulating ribbon elements can result as the voltage is switched.Usually the voltage is switched during the reset interval of thescanning mirror and stray light can reach the screen when the voltage isswitched, thus degrading system contrast. A small amount of light isalso inadvertently directed into the optical system during thistransient which can result in extra reflections and stray light passingthrough the projection optics reaching the display screen. All of thesefactors can degrade system contrast.

Area spatial light imaging modulators such as DLP devices do not exhibitthe same switching effects as linear GEMS, SOM, and GLV devices.However, both area and linear light-modulating devices have a refreshcycle, during which unmodulated light can be inadvertently directed tothe display surface. While the laser itself could be momentarily turnedoff to eliminate stray light during the refresh cycle, such a mode ofoperation is not optimal for existing semiconductor laser devices,compromising wavelength and thermal stability and potentially shorteninglaser lifetimes.

Related to the problem of image contrast is the relative distribution ofdata values over portions of the image. Many types of images include anarea or band of darker values over which there is little perceptibledifference in intensity. Stated differently, such images exhibit a highpercentage of relatively indistinguishable dark values or “dark noise.”Because there is little difference in contrast over such an area, manyfeatures within the image are effectively lost. The skyline of FIG. 1shows one example. In this image, only a silhouette of the skyline isclearly displayed; there is little or no perceptible detail within theskyline band. Buildings, for example, are seen substantially in outline,with almost none of the features within the outline of a buildingvisible. Using conventional display and data value mapping techniques,this poor contrast over such a local area is the best that can beachieved. Even with imaging equipment that is sensitive enough tocapture subtle differences in detail, areas of poor contrast over darkerregions of the image effectively prevent this detail from beingdisplayed.

Thus, it can be seen that improving image contrast relates not only tomethods that help to reduce stray light, but also to methods that canhelp to make details more visible within darker areas and other localareas of an image. There is a need to simultaneously decrease straylight and to improve the contrast of objects in darker areas of an imagein projection apparatus where laser illumination is employed.

SUMMARY OF THE INVENTION

Briefly, according to one aspect of the present invention, there isprovided a method for improving image contrast for a display apparatus,which includes obtaining a frame of image data that includes one or morecode values for each pixel in the frame of image data; analyzing theframe of image data to identify a distribution of dark regions therein;adjusting at least one of the one or more code values for the frame ofimage data according to the distribution of dark regions in the frame ofimage data; and attenuating a brightness level available for the imageframe according to the distribution of dark regions in the frame ofimage data.

It is an advantage of the present invention that it provides an adaptivemethod and apparatus for improving image contrast in a digital imageprojector.

With linear and area light modulators, light attenuation is combinedwith laser blanking, synchronized to the scan to minimize stray light onscreen from light passing through the optics when the lasers are off thescreen. For example, during this time period, when a GEMS device isused, the charge applied to the substrate may be flipped to eliminatehysteresis. Laser blanking minimizes the stray light which occurs duringthe substrate flipping. The laser blanking can be accomplished by eitherswitching the laser off or using an electrooptic modulation device.

The use of an electro-optic modulation device, such as an LC opticalshutter, allows both laser blanking and dynamic illumination control.When an image has a low maximum code value, the contrast of the imagecan be enhanced by making the blacks appear blacker and boosting thedelivered code values of the image proportionally by the amount ofattenuation. Also, the modulation device can be segmented so thatdifferent regions on the screen can have different amounts ofattenuation. This is useful for scenes in which bright areas exist suchas sun-filled sky towards the upper parts of the image and dark detailsat the bottom of the image. As an example, if the maximum intensity of ascene is only 10% of the maximum, code values in the image can beadjusted by a 10× range and the attenuator can be set for 10×attenuation. This can increase the contrast significantly.

The invention and its objects and advantages will become more apparentin the detailed description of the preferred embodiment presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter of the present invention, itis believed that the invention will be better understood from thefollowing description when taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a plan view of an image having a dark region thatconventionally displays with poor contrast;

FIG. 2 is a schematic block diagram that shows components of aprojection apparatus that uses a GEMS imaging modulator in each colorchannel;

FIG. 3A is a logic flow diagram that shows steps for image attenuationand display according to one embodiment;

FIG. 3B, shows a set of gamma curves for representative gamma values;

FIG. 4 is a schematic block diagram that shows components of aprojection apparatus that uses a GEMS imaging modulator in each colorchannel and that is capable of providing improved image contrastaccording to one embodiment;

FIG. 5 is a graph showing relative transmission for a given voltagesignal for a typical EOM device;

FIG. 6 is a perspective schematic view showing the use of control logicprocessor for analyzing image data and controlling projection andbrightness attenuation apparatus accordingly;

FIG. 7A shows an example histogram as conventionally used with a fullbrightness curve;

FIG. 7B shows an example histogram with an increased contrast range andreduced brightness;

FIG. 8 shows imaging results for the image of FIG. 1 using the method ofthe present invention;

FIG. 9A is an example original image having considerable dark pixelcontent;

FIG. 9B is a simulated image showing the results of applying the methodof the present invention to the original image of FIG. 9A;

FIG. 9C is a histogram showing code values for FIG. 9A;

FIG. 9D is a histogram showing code values for adjusted FIG. 9B;

FIG. 9E is a graph that shows normalized accumulated counts versus codevalue obtained from each of the three color channels of the originalimage of FIG. 9A.

FIG. 10A is an example original image having considerable dark pixelcontent;

FIG. 10B is a simulated image showing the results of applying the methodof the present invention to the original image of FIG. 10A;

FIG. 10C is a histogram showing code values for FIG. 10A;

FIG. 10D is a histogram showing code values for adjusted FIG. 10B;

FIG. 10E is a graph that shows normalized accumulated counts versus codevalues obtained from each of the three color channels of the originalimage of FIG. 10A;

FIG. 11 is a timing diagram showing a number of signals used to providelaser blanking between image frames; and

FIG. 12 is a schematic diagram that shows an alternate configuration fora display device according to one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The present description is directed in particular to elements formingpart of, or cooperating more directly with, apparatus in accordance withthe invention. It is to be understood that elements not specificallyshown or described may take various forms well known to those skilled inthe art. Figures shown and described herein are provided in order toillustrate key principles of operation of the present invention and arenot drawn with intent to show actual size or scale. Some exaggerationmay be necessary in order to emphasize relative spatial relationships orprinciples of operation.

In the context of the present invention, a “dark region” in an imageframe is an area of the image frame that contains a substantial numberof pixels below a given threshold code value for darkness within animage. The definition of what constitutes a dark region in anyparticular case depends on a number of factors, including the type ofimaging system and its data representation scheme, the number of pixelsbelow some threshold code value that is identified as dark for thatsystem, and user perception. For the examples given in the presentinvention, low data values, such as values below 50 for an 8-bit systemwith a range of image code values from 0 to 255, are assigned as darkvalues in a projected image.

In color display systems image frame data usually consists of 8, 12, or16 bit integer data provided in the form of an image frame matrix ofsize R by C by 3 where R is the number of rows in the display, C is thenumber of columns in the display and 3 is the number of color planesused to display the image. The location of each image pixel on thedisplay is indicated by the row and column number of the image framedata. The third dimension of the matrix is defined as the color planewith 1 corresponding to the red color plane, 2 to the green color plane,and 3 to the blue color plane. Other sets of color planes can also beused for encoding the data, dependent on the image processing that isdone at the projector, but assume RGB encoding in the discussions thatfollow. In matrix notation if MD was the next image frame data matrixthen IFD(500,600,2)=27 tells that the 500^(th) row and 600^(th) columnhas a green code value of 27 in 8-bit space which would be considereddark by the above criteria. In order for a pixel to be considered darkthe code values for all three color planes should be dark. IfIFD(500,600,1)=255 this would imply that the same pixel has the maximumred code value and is very bright.

It should be noted that an alternate data representation scheme couldhave dark pixels having a value above a certain threshold. The apparatusand methods of the present invention adapt equally to either of thesepossible data arrangements, as well as to pixel data representationsthat use fewer than or more than 8 data bits per pixel. For the methodsand apparatus of the present invention, the distribution of dark codevalues is used to determine, for one or more image frames, both howimage data is adjusted and how output brightness is attenuated.

The apparatus and method of the present invention can be used with adisplay imaging apparatus that employs any of a number of differenttypes of spatial light modulator. By way of an illustrative example, thedescription that follows is directed primarily to a display apparatusthat uses GEMS devices. However, it must be noted that similarapproaches and solutions can also be used for display apparatus that useother linear spatial light modulators or that use area spatial lightmodulators such LCD devices or DLP modulators that use arrays of digitalmicromirror devices.

In the context of the present invention, the term “chip” is used as itis familiarly used by those skilled in the micro-electromechanicaldevice arts. The term chip refers to the one-piece electromechanicalcircuit package that includes one or more light modulator arrays formedon a single substrate, such as the conformal grating devices describedin detail in commonly-assigned U.S. Pat. No. 6,411,425 (Kowarz et al.),mentioned earlier. The GEMS chip not only includes the elongated ribbonelements that form the light-modulating grating for light reflection anddiffraction, but may also include the underlying circuitry that appliesthe electrostatic force that is used to actuate these ribbon elements.In manufacture, the tiny electronic and mechanical components that formthe chip, such as the GEMS chip shown in the Kowarz et al. '425 patent,are fabricated onto a single substrate. The chip package also includessignal leads for interconnection and mounting onto a circuit board orother suitable surface.

Methods of the present invention are particularly well-suited to takeadvantage of the high brightness levels of polarized light availablefrom laser and other solid-state light sources. These methods can alsobe applied where xenon arc lighting or other light sources are used,where the modulated light output is substantially polarized.

Referring to FIG. 2, there is shown a projection display apparatus 10using a GEMS device as linear light imaging modulator in each of threecolor channels, a red color channel 20 r, a green color channel 20 g,and a blue color channel 20 b. For red color modulation, a red lightsource 70 r, typically a laser or laser array, provides illuminationthat is conditioned through a spherical lens 72 r and a cylindrical lens74 r and directed towards a turning mirror 82 r. Light reflected fromturning mirror 82 r is modulated by diffraction at a linear lightimaging modulator 85 r, shown and described herein as anelectromechanical grating light modulator. Modulated diffracted lightfrom linear light imaging modulator 85 r is diffracted past turningmirror 82 r and to a color combiner 100, such as an X-cube or otherdichroic combiner. The modulated line of light from color combiner 100is then directed by a lens 75, through an optional cross-order filter(not shown), to a scanning element 77 for projection onto a displaysurface 90. Scanning element 77 can be a scanning mirror commonlyreferred to as a galvanometer or a galvo or other suitablelight-redirecting scanning element, such as a rotating prism or polygonor an apparatus having one or more coupled reflective surfaces, whichapparatus, in turn, directs the incident modulated lines of light forforming 2D images toward display surface 90. Green color modulation usesa similar set of components for providing light to color combiner 100,with a green light source 70 g, typically a laser or laser array,providing illumination through a spherical lens 72 g and a lenscylindrical 74 g and directed towards a turning mirror 82 g. Lightreflected from turning mirror 82 g is modulated by diffraction at anelectromechanical grating light modulator that serves as a linear lightimaging modulator 85 g. Modulated diffracted light from linear lightimaging modulator 85 g is diffracted past turning mirror 82 g and tocolor combiner 100. Similarly, blue light source 70 b, typically a laseror laser array, provides illumination through a spherical lens 72 b anda cylindrical lens 74 b and directs light towards a turning mirror 82 b.Light reflected from turning mirror 82 b is modulated by diffraction atan electromechanical grating light modulator that serves as a linearlight imaging modulator 85 b, is diffracted past turning mirror 82 b,and is sent as a line of light to color combiner 100.

Embodiments of the present invention provide improved image contrast bycombining a code value adjustment with brightness attenuation of themodulated light. Brightness attenuation is achieved by interposing oneor more electro-optical light modulators into the path of modulatedlight and selectively controlling the electro-optical modulator(s) toattenuate some percentage, but not all, of the light. Attenuationdecreases the range of available light, but allows the same number ofdata values for light intensity to be used. In addition, adjustments tothe display gamma curve are also made, to allow for non-linearities indisplay output and in viewer response. As a result of these combinedadjustments, contrast within the more limited light range can beenhanced, increasing the visibility of details that were not previouslyperceptible when considering the full range of available light.

The logic flow diagram of FIG. 3A shows the sequence of steps used inorder to obtain improved contrast for an image frame according to oneembodiment of the present invention. An obtain image frame step S200begins the process by obtaining the data for a single frame of theimage. A histogram analysis step S210 then generates and analyzes ahistogram of the image frame in order to determine whether or not theimage meets the necessary criteria for contrast improvement using thismethod. Criteria for suitability are empirically determined but arechiefly based upon the distribution of dark image code values. In oneembodiment, this is determined by whether or not dark code value bandsor portions of significant size can be detected within the image.Histogram analysis provides one type of pixel value distribution datathat can be a particularly useful tool for making this determination.Using histogram analysis, an accumulated count of pixels below aspecific threshold value can also be used for determining whether or notthe image frame has significant dark content and for quantifying thatcontent in some way. In general histogram analysis needs to be performedon all three color planes in a color image, and the accumulated count ofpixels below a specific threshold value criteria must apply to all threecolor planes in order to assess that the image frame has significantdark content. More sophisticated utilities, such as image processingalgorithms that check for groupings of dark pixels within a region, canalso be used to provide information about the distribution of dark pixelvalues.

A test step S220 then evaluates the image histogram analyzed in stepS210 to determine whether or not the image meets the criteria forcontrast adjustment along one of the processing paths shown collectivelyas processing step S230, or is displayed without attenuation and withthe standard gamma curve applied. The gamma setting is applied in agamma application step S240. If the criteria are met, a discrete levelof brightness attenuation is identified, shown by way of example aseither 2×, 5×, or 10× in FIG. 3A. As the scene content gets darker, itis desirable to use greater pixel intensity shifting at lower imagepixel code values relative to that employed at higher image pixel codevalues. As an example a 2× brightness-attenuated image would bedisplayed with a gamma of 0.8 relative to the original image codevalues, a 5× attenuated image would be displayed with a gamma of 0.65and a 10× attenuated image would be displayed with a gamma of 0.5. Adisplay step S250 then displays the processed image. It can be notedthat display step S250 can be executed immediately, so that the processshown in FIG. 3A executes as a part of the standard image processingchain for a projector or other apparatus, for example. Alternately, thecomputed attenuation and gamma information for an image frame can bestored for subsequent use and for display at a later time, using any ofa number of display types.

Gamma correction is an operation that is well known in image displayprocessing, used to encode and present image data in a form that is wellsuited to human perception and display characteristics. As applied inthe apparatus and methods of the present invention, gamma is anadjustment applied to an input code value. Referring to FIG. 3B, thereis shown a set of gamma curves for representative gamma values. For thisexample, 8-bit code values (0-255) are used. For reference, a gamma of1.0 is linear, imparting no change to an input code value. Theconventional display gamma for a typical DCI (Digital CinemaInitiatives) projector for digital cinema is 2.6, shown in bold in FIG.3B. This gamma correction is applied by the projector or other displaydevice to all input image data. In the method of the present invention,brightness attenuation is provided when the gamma value is less than 1,as shown in the logic flow diagram of FIG. 3A.

The output gamma code value (GCV) is given by the expression:GCV=MCV*(OCV/MCV)^(γ)where MCV is the maximum code value, OCV is the original code value andγ is the value of gamma. In a three color system, if the input image isprovided in RGB coordinate space, this is calculated by creating a lookup table of new gamma code values GCV vs. original code values OCV andapplying the formula to each of the three color planes for each pixel inthe image. The MCV is 255 (2⁸−1) for 8 bit systems, and 4095 (2¹²−1) for12-bit systems.

Histogram analysis can be particularly efficient when a value range inthe image histogram can be spatially correlated with a horizontal orvertical band or other specific area of the image. For the image shownearlier in FIG. 1, for example, the dark value range in the imagehistogram clearly maps to the buildings and other features along a lowerband of the image.

Brightness Attenuation Components and Operation

The schematic block diagram of FIG. 4 shows how brightness is attenuatedby an attenuation apparatus 38 in one embodiment of a display apparatus120 of the present invention. Attenuation apparatus 38 has two types ofcomponents: (i) a phase-modulating electro-optical modulator 64 that isa light polarization modulator disposed in the path of the combinedmodulated light, and (ii) an analyzer 66. When actuated by a variablecontrol signal, electro-optical modulator (EOM) 64 changes the phase ofthe incident light, effectively rotating the polarization of theincident light by a corresponding number of degrees. Analyzer 66 has itstransmission axis in parallel with light sources 70 r, 70 g, and 70 b,so that modulated light passes through to display surface 90. Thus,analyzer 66 blocks a portion of the light that is modulated when EOM 64is actuated.

In one embodiment, electro-optical modulator EOM 64 is an electroopticpolarization rotator, such as an LF Series Optical Shutter from BNS,Sweden. This device is conventionally driven with a 2-5 kHz square waveof up to ±20 V. When there is no drive signal applied, the liquidcrystal molecules in this EOM device rotate the polarization of theinput light by 90°. When an AC signal with a high amplitude is suppliedto the polarization rotator, the liquid crystal molecules realign and nolonger rotate the polarization of the incoming light. Alternately, themode of operation of the phase-modulating electro-optic modulator 64 canbe reversed, so that, when not actuated by a control signal, it changesthe phase of the incident light by 90 degrees and, when actuated, itcauses 0 degree change to the polarization.

In order for the EOM device to function properly, care is taken toprovide a zero net DC bias to the liquid crystal layer. This isaccomplished by applying a high frequency square wave during the closedstate, typically between 2-5 kHz, as noted.

EOM devices of this type typically have different rise and fall responsetimes. Rise time is defined as the amount of time for the rotator toswitch from the energized state (0° rotation) to the de-energized state(90° rotation), measured from 10% to 90% of full modulation. Fall timeis the amount of time for the rotator to switch from the de-energizedstate (90° rotation) to the energized state (0° rotation), measured from90% to full 10% of full modulation. The rise time is generally fixed bythe design of the polarization rotator and varies somewhat as a functionof temperature. The fall time varies as a function of temperature aswell, but it is also controlled by altering the amplitude of the ACdrive signal. Higher amplitudes generally provide faster fall times.

With EOM devices of this type, the relative amount of rotation of theincoming light is also controlled by the amplitude of the applied ACdrive signal. A characteristic curve showing percent light output as afunction of applied AC drive voltage is provided in FIG. 5. As thisgraph shows, the active region of the EOM device enables control of thelevel of light transmission throughput. For the device shown in FIG. 5,a drive voltage of 4 volts provides a 2× brightness attenuation, a drivevoltage of 5 volts provides a 5× attenuation, a drive voltage of 5.9volts provides a 10× attenuation and a drive voltage greater than 12volts provides a near 100% attenuation with an on/off ratio greater than500 to 1. Embodiments of the present invention allow use of thiscapability to provide a dynamically controllable level of modulatedlight, controlled according to image content.

FIG. 6 shows how display apparatus 120 of the present invention handlesthe evening cityscape image of FIG. 1 in one embodiment. Here, two EOMs64 a and 64 b are shown being used for image display, each correspondingto a horizontal band or region 42 or 44 of the displayed image.Alternately; where control of various areas of the EOM itself isavailable, a single EOM could be used for the entire image frame,adaptive to the arrangement of image content. Depending on the type ofEOM used, local control of brightness attenuation could even be appliedto any portion or region of the image, even including brightnessattenuation applied over one or more individual pixels or clusters ofpixels. One consideration, however, in applying different attenuationlevels to different portions of the image frame relates to transitionsbetween portions, in order to minimize imaging anomalies. In yet anotherembodiment only one EOM 64 could be used for the entire image. A controllogic processor 56 accepts and analyzes and conditions the input imagedata that goes to the light modulators in display apparatus 120 andperforms the attenuation control and image data manipulation requiredfor embodiments of the present invention. Control logic processor 56controls the operation of EOMs 64 a and 64 b to provide the neededbrightness attenuation, according to the analysis of the image frame.

Referring back to the logic flow diagram of FIG. 3A, image analysisproceeds by obtaining a histogram or other type of representativedistribution for data values in each defined horizontal or verticalregion of the image or in the complete image frame (step S210). Theschematic diagram of FIG. 7A shows a representative histogram for theskyline content of the image shown as region 44. As can be seen fromthis image, the bulk of values from the histogram are grouped within aninterval A that corresponds to a small portion of the brightness valuesavailable. In the example of FIG. 7A, most of the histogram values forregion 44 are within brightness levels not exceeding 0.4 normalizedbrightness. This means that a significant number of code values areunused or minimally populated, as shown. A significant portion of thedarker pixels lie within the “dark noise” range, and thus providerelatively poor contrast over darker portions of the image. As a resultof this distribution, very little contrast is achievable in the darkerareas of the image.

FIG. 7B shows schematically how the method of the present inventionimproves image contrast. First, the brightness level available isattenuated. In the example shown, only 0.6 of the brightness is nowavailable due to modulation of EOM 64 b (FIG. 6). However, thisbrightness attenuation allows previously little-used or unused codevalue space to be employed for contrast improvement. In effect, thisexpands interval A to provide interval A′ as shown in FIG. 7B. Now, therange of brightness in the image is the same, but the number ofbrightness levels that are now available within this range isdramatically increased. The proportion of pixels that are now consideredto be “dark noise” is greatly reduced. This leads to improved imagecontrast over the dark areas of this example, as shown in FIG. 8. Forthe image first shown in FIG. 1, two treatments of a portion of thecityscape are shown in FIG. 8. In a first image portion at B1,conventional treatment of the image for display is provided, withrelatively poor contrast as a result. In the image portion at B2, themethod of the present invention has been applied, with combinedattenuation of the available brightness level and expansion of the rangeof available code values as described earlier with reference to FIG. 7B.This results in a dramatic improvement in image contrast. Again, it canbe noted that the extreme light and dark tones of the example image areunchanged between image portions B1 and B2. However, the range ofavailable tones is greatly expanded using the attenuation of overallbrightness and range adjustment provided in embodiments of the presentinvention.

A number of observations can be made based on the example described withreference to FIGS. 6-8, including the following:

-   -   (i) A combination of both brightness attenuation and        corresponding image data adjustment is provided by control logic        processor 56 in order to achieve the resulting contrast        improvement on an image-by-image basis. Darker regions can be in        horizontal bands, vertical bands, or distributed with other        geometries. For embodiments with more than a single EOM device        in the image path, the flexibility of this method can be        affected by how regions of the display surface are assigned        relative to EOM devices.    -   (ii) Histogram manipulation expands the number of code values        available for darker portions of the image, but this is at the        cost of reducing the number of values available for brighter        portions. The relative proportion of dark code values and the        overall distribution of code values for an image frame can be        factors in determining how to apply the method of the present        invention.    -   (iii) Control logic processor 56 (FIG. 6) may be a computer        workstation, microprocessor, or other type of computer device        that performs image analysis and display control. Optionally, a        separate microprocessor or other control logic device can be        used specifically for EOM device control.    -   (iv) In practice, it may be cumbersome an unnecessary to provide        a continuous range of brightness levels. Instead, a set of        discrete brightness attenuation levels is typically used such as        2×, 5× and 10×, and would be set with corresponding gammas of        0.8, 0.65 and 0.5 respectively.    -   (v) Gamma adjustment to the code values is also provided as the        overall brightness level is reduced by EOM control. Gamma        adjustment curves are generally specific to a particular imaging        system type.

FIGS. 9A through 9D show an example image to which the method of thepresent invention is applied. FIG. 9A shows the original outdoor image,taken near dusk. Features below the horizon are barely perceptible inthe original figure. By way of simulation, FIG. 9B shows the results ofa combination of attenuated brightness level and adjusted code valuesfor the same image. Features below the horizon are now more clearlyperceptible, as can be seen from the enlarged portion along the rightside of each figure.

FIGS. 9C and 9D show the original and adjusted histograms, respectively,for FIGS. 9A and 9B. A brightness scale appears at the bottom of eachhistogram. As is clear from comparing the histograms, FIG. 9D expandsthe relative range of the code values allotted to the darker codevalues. A gamma value of 0.8 is applied in this example. A correspondingattenuation to image brightness, 50% in this example, allocates anexpanded data range for these darker values.

The graph of FIG. 9E shows, in normalized form, the accumulated countsversus code values obtained from each of the three color channels of theoriginal image of FIG. 9A. As is clearly seen from this graph, the vastmajority of pixel code values in all three color channels for theoriginal image of FIG. 10A are below about 160; with less than 1% of thepixel code values above this level. Also, more than 50% of the pixelcode values are below 80 in all three color channels.

As a more pronounced example, FIGS. 10A through 10D show the method ofthe present invention applied to an image of the same scene as in FIG.9A, but taken further toward nightfall. FIG. 10A shows the originalimage, in which the sky is barely discernable from the land and featuresof the landscape are not perceptible. FIG. 10B shows the resulting imageafter applying the combination of reduced brightness, here attenuated to10% of the full range, plus adjustment of code values for the darkerpixels, here with a gamma of 0.5. FIG. 10C shows a portion of the greenchannel, here, the bottom half, for the original image. FIG. 10D thenshows the histogram for the same data with the 10% brightnessattenuation and 0.5 gamma applied.

The graph of FIG. 10E shows, in normalized form, the accumulated countsfor code values obtained from the original image of FIG. 10A. As isclearly seen from this graph, the vast majority of pixel code values forthe original image of FIG. 10A are below about 36 in all three colorchannels; very few code values above this level are in this dark image.This is one type of information that can be used as a criterion fordetermining that there are substantial dark regions in an image and forascertaining both the needed level of brightness attenuation and thecode value adjustment to be used, such as applying a given gamma value.

Laser Blanking Between Frames

The method of the present invention can be used in conjunction withtechniques that provide laser blanking between image frames. As wasnoted earlier with reference to the Kowarz et al. '481 patent,performance of GLV or GEMS ribbons and other electromechanicalmodulators can be degraded both by long-term actuation and by chargedeposition in repeated actuation, which causes “stiction” and othernegative performance effects. Therefore, in practice, the GEMS device isneither driven with too many pulses of the same polarity nor are thepixels driven continuously within an image frame. Because of this,various timing schemes continually reverse the actuation voltagepolarity using a grounded substrate and a bipolar high voltage driverfor each pixel or, alternately, repeatedly switch the substrate biasvoltage itself. After each image frame is displayed, the substrate isdriven to the opposite voltage polarity. The effect of either of thesesolutions on transient light is the same; there is some transient effectthat can result in the unintended leakage of light during the framerefresh cycle.

By way of example, the timing diagram FIG. 11 shows the temporalrelationship of galvo drive signal 30 and a switched substrate biasvoltage 50 and shows the effect of these switching signals as it relatesto light delivery and light leakage for a GEMS or GLV projector. A lighttiming signal 60 (dotted curve in light to screen graph) shows the lightON or enabled, during writing portion A of the scanning cycle as theimage is being scanned to the display. However, during retrace portion Bof the scanning cycle, an output light transient is detected due tounintended ribbon element movement. As shown in FIG. 11, light transientrepeats at each transition of substrate bias voltage 50, that is, onceduring each retrace portion B of the galvo mirror. The time intervalbetween times t1, t2, and t3 as shown is approximately 16 msec in oneembodiment.

The light to screen waveform 60′ (solid curve in light to screen graph)is adapted to change the available brightness from frame to frame, asshown in the example of FIG. 11. The timing diagram of FIG. 11 shows thetiming of an EOM signal 68′ (solid curve in EO modulator state graph)relative to galvo drive signal 30 and bias voltage 50, and shows how EOMactuation impacts light timing signal 60′. The light transient betweenimage frames is suppressed, eliminating this unwanted light from theoutput. Plus, the brightness attenuation and data manipulation of thepresent invention is performed, giving EOM devices 64 a and 64 b a dualpurpose in display apparatus 120. The timing diagram of FIG. 11 alsoshows the EOM signal 68 (dotted curve in EO modulator state graph) whichwould be used when not attenuating during image frames, but includesblanking in between frames to suppress the light transient between imageframes.

The alternate embodiment of FIG. 12 shows an embodiment of displayapparatus 120 with an EOM 64 r, 64 g, 64 b in each color channel and asingle analyzer 66 at the output. Alternately, each channel may have ananalyzer and an EOM. It may be sufficient for some applications toprovide light modulation and laser blanking to only some of the colorchannels, such as only to the green color channel, which has the mostpronounced effect on luminance.

In order to switch at sufficiently high rates for laser blanking, EOM 64a, 64 b must have a fast response time. Until recently, EOM devices werenot able to respond quickly enough for the display apparatus timingdescribed with reference to FIG. 11. However, more recent improvementsin EOM speed and overall performance are now making it possible to usethese devices for the laser blanking required between image frames aswell as for attenuation within the frames. EOMs that can be used forlaser blanking as described with reference to FIGS. 4 and 10 can be anysuitable types of devices, such as VX series modulators from BoulderNonlinear Systems, Inc., Lafayette, Colo.

The apparatus and method of the present invention thus provide a dynamicillumination control for an image projection apparatus that can help toimprove image contrast for individual image frames. When an image has alow maximum code value, the contrast of the image can be enhanced bymaking the blacks appear blacker and boosting the delivered code valuesof the image proportionally by the amount of attenuation. Also themodulation device can be segmented so that different regions on thescreen can have different amounts of attenuation. This is useful forscenes in which bright areas exist such as sunny sky at the top and darkdetails at the bottom of the image. As an example, if the maximumintensity of a scene is only 10% of the maximum code values in the imagecan be adjusted by a 10× range and the attenuator can be set for 10×attenuation. This can significantly increase image contrast. Moreover, avariable optical shutter, such as an EOM, can provide both brightnessattenuation during projection of an image frame and laser blankingbetween frames, wherein the laser blanking is synchronized to the framegeneration sequence to minimize stray light on screen from light passingthrough the optics, such as when the lasers are off the screen forscanning GEMS- and GLV-based systems.

The apparatus and methods of the present invention have been describedprimarily with reference to projection systems that use GEMS, GLV, orother scanned linear light modulators. It must be noted, however, thatthe apparatus and methods of the present invention are also applicableto projection apparatus that employ area spatial light modulators, suchas LCD modulators or micromirror arrays, such as those used in DLPdevices. It should also be noted that the method of the presentinvention can be applied to all composite colors of a color projector orto one or more color channels.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention.

PARTS LIST

-   10 display apparatus-   20 r red color channel-   20 g green color channel-   20 b blue color channel-   30 drive signal-   38 attenuation apparatus-   42 region-   44 region-   50 bias voltage-   56 control logic processor-   60 timing signal-   60′ timing signal-   64 electro-optical modulator-   64 a electro-optical modulator-   64 r electro-optical modulator-   64 g electro-optical modulator-   64 b electro-optical modulator-   66 analyzer-   68 signal-   68′ signal-   70 r red light source-   70 g green light source-   70 b blue light source-   72 r spherical lens, red-   72 g spherical lens, green-   72 b spherical lens, blue-   74 r cylindrical lens, red-   74 g cylindrical lens, green-   74 b cylindrical lens, blue-   75 lens-   77 scanning element-   82 r turning mirror, red-   82 g turning mirror, green-   82 b turning mirror, blue-   85 r linear light modulator-   85 g linear light modulator-   85 b linear light modulator-   90 display surface-   100 color combiner-   120 display apparatus-   S200 obtain image frame step-   S210 histogram analysis step-   S220 test step-   S230 processing step-   S240 gamma application step-   S250 display step-   A interval-   A′ interval-   B1 image portion-   B2 image portion-   t1 time-   t2 time-   t3 time

1. A method for improving image contrast for a display apparatuscomprising: obtaining a frame of image data said image data comprisingone or more code values for each pixel in the frame of image data;analyzing the frame of image data to identify a distribution of darkregions therein; adjusting at least one of the one or more code valuesfor the frame of image data according to the distribution of darkregions in the frame of image data; attenuating a brightness levelavailable for the image frame according to the distribution of darkregions in the frame of image data; and wherein the frame of image dataincludes a first portion and a second portion of image data from theframe and analyzing a distribution of dark regions within the first andsecond portions, adjusting at least one code value for each of the firstand second portions, attenuating the brightness level of each of thefirst and second portions, and displaying the first and second portionsof image data, wherein a brightness level attenuation for the secondportion differs from the brightness level available for the firstportion.
 2. The method of claim 1 wherein analyzing the distribution ofdark regions within the frame of image data comprises evaluating animage histogram.
 3. The method of claim 1 wherein the frame of imagedata comprises the complete image.
 4. The method of claim 1 wherein theframe of image data comprises a vertical or horizontal portion of theimage.
 5. The method of claim 1 wherein attenuating the brightness levelcomprises disposing an electro-optical modulator in the path ofmodulated light in the display apparatus.
 6. The method of claim 1wherein attenuating the brightness level comprises setting brightness toone of a set of fixed values.
 7. The method of claim 1 furthercomprising attenuating output light after displaying the frame of imagedata.
 8. The method of claim 1 further comprising displaying the frameof image data with the attenuated brightness level and adjusted codevalues.
 9. The method of claim 1 wherein attenuating the brightnesslevel applies the same attenuation to the entire image frame.
 10. Themethod of claim 1 wherein attenuating the brightness level appliesdifferent attenuation over different portions of the image frame.
 11. Amethod for improving image contrast for a display apparatus comprising:obtaining a first frame of image data said image data comprising one ormore code values for each pixel in the first frame of image data;obtaining a second frame of image data said image data comprising one ormore code values for each pixel in the second frame of image data;analyzing the first and second frames of image data to identify adistribution of dark regions therein; adjusting at least one of the oneor more code values for the first frame of image data according to thedistribution of dark regions in the first frame of image data andattenuating a brightness level available for the first image frameaccording to the distribution of dark regions in the first frame ofimage data; adjusting at least one of the one or more code values forthe second frame of image data according to the distribution of darkregions in the second frame of image data and attenuating a brightnesslevel available for the second image frame according to the distributionof dark regions in the second frame of image data; and attenuating thebrightness level between display of the first and second image frames.